The present invention relates to a control system for variable capacity compressors, and more particularly to a method of controlling two or more variable capacity compressors arranged in parallel.
An existing method of controlling fixed capacity compressors in parallel systems is referred to as “Round-Robin” cycling. In “Round-Robin” cycling, the first compressor turned on as the system load increases is also the first compressor turned off as the system load decreases. For example, in a system including three compressors, a first compressor is turned on and runs at a fixed capacity when the load increases from zero. As the load continues to increase, the second compressor is turned on and runs at a fixed capacity followed by the third to reach the maximum capacity of the compressors. When the load begins to decrease with all three compressors running, the first compressor is initially stopped, and the second and third remain running at their respective fixed capacities. Continued decreases in the load will cause the second compressor to be turned off leaving the third as the last running compressor. Finally, the third compressor will be turned off due to further reduction in the load. This known method does not utilize variable capacity compressors, and therefore the incremental unit of adjustment of the capacity of the entire compressor bank is limited to the full fixed capacity of a single compressor.
Another existing method of controlling compressors in parallel systems includes a single variable capacity compressor in addition to the multiple fixed capacity compressors. In this method, the single variable capacity compressor is used in combination with the “Round-Robin” cycling to provide increased incremental control between activation and deactivation of the fixed capacity compressors thereby allowing the entire bank of compressors to more accurately match the load. For example, a variable capacity compressor could be used with the three-compressor “Round-Robin” system described above. In this system, rather than starting a new compressor when the load exceeds the capacity of the running compressors, the variable capacity compressor is adjusted to match the load. Likewise, rather than shutting down a compressor when the load is lower than the capacity of the running compressors (including the variable compressor) the capacity of the variable capacity compressor is decreased to match the load. Only when the capacity percentage of the variable capacity compressor falls outside of an acceptable capacity range is a new fixed capacity compressor started or a running fixed capacity compressor turned off. In this circumstance, the variable capacity compressor is then adjusted to compensate for the added or removed fixed capacity compressor to match the load.